How to calculate obscuration of the light by small particles

Hi there,
I am trying to find the number of fine particles (micron size) by obscuration. There are a light source and a photo-detector which particles are injected between them. I know the size of my particles. How can I find how many particles are passing through the light by obscuration of light?

This is an entire field of study in itself. It's important for the study of growth of aquatic plants (which are affected by the amount of light penetrating to a given depth) and for satellite imaging of oceans.

My dad wrote a book about it, that I understand is highly regarded in the industry:

The intensity of light reaching the detector depends on
- the quantity of light emitted
- the density of particles in between
- the shape of the particles
- the nature of the particles
- the angle of the light source to the receptor

Both scattering and absorption needs to be taken into account, and in some cases also reflection. Monte Carlo simulation of photon paths is the standard way to develop distributions of light intensity at the receptor.

Given the complexities that andrewkirk pointed out, perhaps the easiest thing to do is to work this out experimentally.

If you can inject a known quantity of particles into your system (shouldn't be too hard -- if you know what they're made of and their size, you can do this by mass), you can make measurements of the obscuration of the light for different amounts of particles (0 to as many as you think is appropriate), and construct a Number vs Intensity curve. Then, if you keep the setup static, you should be able to use that curve for an arbitrary number of particles.

Hi there,
I am trying to find the number of fine particles (micron size) by obscuration. There are a light source and a photo-detector which particles are injected between them. I know the size of my particles. How can I find how many particles are passing through the light by obscuration of light?

As andrewkirk mentioned, there are a lot of complexities. The usual approach is to calibrate your system using known concentrations of (similar) particles.